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Version:

August 20, 2016:
Revised: v1.0

The Cardboard ELSIE LC Meter

Shortly
after arriving to start a new job and keen on keeping up my hobby, I
needed to measure inductors and capacitors. I built this little device
using the only parts I could find. It's based on the ELSIE LC meter
designed by Steve KD1JV.

Introduction

When I
got a new job in the Middle East, and being familiar with the
situation, I recognized the importance of having something to do during
my spare time. Despite evidence to the contrary, there is only so much
time you can spend watching movies and shopping in malls.

One
of the things I decided to do was to keep up my electronics hobby in
some limited way. To do that, I decided to take just a handful of parts
and a few small items of test equipment. That amounted to a small
cardboard box containing those parts, a cheap soldering iron, a small homemade variable power supply, and a low cost multimeter.

After
three months or so, I decided that I needed a few other items to
make things a little easier: A transistor tester, a cheap digital
oscilloscope, and an LC meter. Within a few months, I’d built the transistor tester,
and I’d managed to bring a moderately priced scope back in my bag after
a brief trip to the UK. However, the LC meter still presented a
problem. (This was before all of those Chinese-made LC meters became
available on the web….)

Almost every design I looked at
required an LM311. This was used to in the LC meter's oscillator, its
frequency being set by the inductor or capacitor being tested. A cheap
microcontroller, most often a PIC16F48, was most often used
to count that frequency, and then calculate and display the
resulting value of the inductor or capacitor on an LCD. Problem
was, my small parts collection did not include any LM311 chips, or
a PIC, or an LCD.

While searching for an alternative
approach, I realized that I could probably make a suitable oscillator
by using one of several 74HC04 hex inverter SMD chips I had in my
little box of parts. A few tests confirmed that approach was reasonably
sound.

The LCD was the next problem. The discovery of
a low cost design from Steve Weber KD1JV which used a Morse code and a
beeper to report L or C values was timely. It
used an ATtiny-compatible microprocessor which was no longer
manufactured. However, I did have an ATtiny chip in my little parts box
that was a recommended replacement.

Steve also described
an upgraded version which added a small 7-segment LED display. I also
had one of those in my small pile of parts, and even a 78L05 regulator.
Actually, my LED display was a “common cathode” type rather than the
common anode type specified in the Steve’s design, but the software
changes required to sort that out were clearly going to be minimal.

Finally,
the 14.318MHz crystal I had in my box of bits was on not the same
frequency as the 8.192MHz crystal used in the original design, but I
was again quite confident I could figure out the software changes
required to sort that problem out. So, I was good to go.

Uh,
almost good to go. I still had no way to actually build the design – I
had no prototyping board nor any PCB scraps. Nothing. So how could I
build the LC meter?

Scouting about, I stumbled across a
scrap of cardboard. So, yes, my LC meter ended up being built on a
scrap of thin cardboard. “Needs must”, as they say.

Here’s my revised schematic reflecting the parts I had available:

Construction

I
built the 74HC04 oscillator section on top of the card, pushing a
couple of holes through the card to anchor several of the leaded parts.
The IC socket, crystal, the LED display and the other parts were all
mounted on the card too, again, making holes with a pin and soldering
the connections on the underside of the card.

The wires going to the 9V battery clip were anchored by passing them through an extra pair of holes.

After
programming the ATtiny2313 and testing it, I glued the cardboard
circuit to a backing piece of corregated card. This insulated the
connections on the back of the card and added additional rigidity to
the meter. You can see the result in the photo at the top of the page.

Operation

The
resulting LC meter worked remarkably well, and I used it for about
three years. It reports every result using six digits, sequentially
displaying these digits in turn on the LED display and sounding out the
digits in Morse, in picofarads for capacitance and in microhenries for
inductors. After
all that time, I had managed to buy a few 16x2 LCD displays. I
subsequently designed and built several LC meters which displayed
the inductance or capacitance values more traditionally, directly on an
LCD. Since they used I2C LCD displays which I happened to get reasonbly
cheaply, they required some new software which I wrote from scratch.
I'll describe these at some future date.

Conclusions

This
cheap and basic design made it possible for me to continue my hobby
in my limited space without much cost. I've described it here
just to show how it is possible to build things when you only have a
few parts, when necessary.

Download:

Cardboard_LC_Meter: This zip
file includes the modified assembly source code (modified from
Steve KD1JV's original software to suit the ATtiny2313, different
crystal and changed display type), and the HEX file for those just
wanting to
program an ATtiny2313 directly.